1. Field of Invention
This invention relates to a substrate for use in printed circuit boards, and more particularly to a novel process for producing the substrate.
2. Related Art
Today, printed circuit boards are used in many fields such as computers, communication devices, domestic appliances, as a baseboard for receiving IC and LSI. As the substrate for such printed circuit boards, laminated boards are used. They comprise a woven fabric of glass fiber or paper or the like reinforcing material saturated with a matrix of epoxy resin, polyester resin, phenolic resin or polyimide resin. However, printed circuit boards using these laminates as substrate are disadvantageous because combining the starting woven glass fiber fabric with the matrix resin to form finish a printed circuit board capable of receiving IC or LSI is a very long and tedious process. For example, in the case of a printed circuit board made of a woven glass fiber fabric and epoxy resin, the woven fabric is first impregnated with epoxy resin and dried to prepare a preimpregnated sheet. In the next step, the desired number of preimpregnated sheets are laminated and a copper foil or the like superposed thereon. The whole construction is then hot-pressed to prepare the laminate board. Thereafter, a resist is formed and the board is etched to form the circuit, after which holes are drilled to receive the IC or VLI.
In addition to producing a printed circuit board using a laminate board, it has also been proposed to make printed circuit board by injection molding.
In the latter process, a thermoplastic resin is used as the matrix resin, and a previously formed circuit is transferred onto a transfer sheet simultaneously with the injection molding, whereby a circuit is formed on the molded product during the molding step.
According to the process of the invention, printed circuit boards can be obtained which may be not only planar, but also steric in shape. Although printed circuit boards prepared in this manner are limited in use because of the performance of the board itself and the limitations on the formable circuit patterns, this process has an important advantage in that the production cycle can be significantly shortened. Thus, this process may be adopted in new products in the continually expanding fields in which printed circuit boards are used.
On the other hand, photosteric lithography has been proposed as a method for making formed products of plastics, although not for printed circuit boards. For example, this process has been proposed in Japanese Patent Application Kokai (Laid Open) No. 62-101408, Japanese Patent Application Kokai (Laid-Open) No. 63-251227, Japanese Patent Publication No. 63-40650, etc. According to this process, a photo-curable resin is introduced into a vessel, and the resin surface is irradiated with a laser beam to cure the resin as desired. After shifting the resin surface, the laser beam is again projected to form a cured resin layer over the previously cured resin layer. By repeating this procedure, while altering the pattern of the projected laser beam, products having a variety of steric shapes can be obtained.
The pattern to be projected may be designed by CAD and the desired steric image cut by the laser beam into the thin layers. A personal computer or the like can guide a projecting part and the resin-container so as to automatically move and trace the thin-layered patterns to be cut. A laser beam is the type of beam considered most suitable for the purpose because of its high energy level and low convergence. As a modification of the above-mentioned photosteric modelling process, a planar ray, such as an ultraviolet ray, may be projected through a film mask.
As has been mentioned above, the conventional process for obtaining printed circuit board by the use of a laminated board requires long and tedious procedures, and therefore cannot be quickly modified for small production runs and for responding to the rapidly changing needs of consumer products.
On the other hand, the process by injection molding is very time consuming and costly for designing and making the die. Further, since it necessitates the use of a die, it is unsuitable for many kinds of small quantity production projects.